The present invention relates generally to the implementation of heating, ventilation and air conditioning (HVAC) systems for controlling the air quality within one or more rooms of a building. In particular, the present invention relates to the control of HVAC systems to maintain air quality within desired temperature and carbon dioxide level parameters.
Heating, ventilation and air-conditioning (HVAC) systems are widely used to control the air quality within rooms of buildings. A variety of different parameters concerning the air quality can be controlled. Two commonly controlled parameters include the air temperature within the room(s) and the carbon dioxide (CO2) levels within the room(s).
In typical HVAC systems, air quality within a room is controlled by mixing some fresh air (outside air) with some return air (existing room air), heating or cooling the mixture, and then providing that heated or cooled mixture back to the room as discharge air. Control of the ratio of the fresh air component to the return air component within the room is a key mechanism for controlling air quality.
With respect to controlling the CO2 levels within a room, the number of people in a room is the major source of CO2 generation. HVAC systems that employ Demand Control Ventilation (DCV) vary the amount of fresh air provided to the room in order to maintain the CO2 at or below desired levels, despite changes in the number of occupants within the room.
The proportion of fresh air within the discharge air can also be varied to influence the room temperature.
For example, when the fresh air temperature is lower than an economizer switch-over temperature setpoint, additional fresh air can be added to the return air to reduce the overall temperature of the discharge air and the room.
Although the relative mixture of the fresh air and return air components in the discharge air can be varied widely to control temperature and CO2 levels, the temperature of the discharge air cannot be reduced below a certain low limit without reducing the comfort of occupants within the room or causing excessive down draft. Consequently, the HVAC system typically must monitor the temperature of the discharge air and make sure that it does not fall too low.
Under many circumstances, an HVAC system can control (or at least influence) both the temperature of a room and the CO2 levels within the room simultaneously by varying the amount of fresh air being provided to the room. However, when CO2 levels are high but outdoor temperatures are low, control of both parameters simultaneously can become difficult. Because CO2 levels are high, presumably because of a high number of occupants within the room, a greater amount of fresh air is desirable to reduce the CO2 levels. At the same time, because the outdoor temperatures are low, large amounts of fresh air can overly reduce the discharge air temperature and create discomfort for the occupants.
More specifically, as long as the HVAC system is able to sufficiently heat the mixture of the fresh air and the return air to keep the discharge air temperature from falling below a desired discharge air temperature (DAT) setpoint, desired control of both the room temperature and the CO2 levels is possible. However, if the discharge air temperature falls below the DAT setpoint but the HVAC system is providing heat at or above its capacity, desired control of both the room temperature and the CO2 levels is limited.
It would therefore be advantageous if an HVAC system and method were developed that enabled optimal control of the air quality within a room when (i) high levels of fresh air are desirable in order to reduce excessive CO2 levels due to a large number of occupants in the room, and yet (ii) the fresh air temperature is sufficiently low that the HVAC system cannot provide sufficient heat to warm up the discharge air temperature to above a DAT setpoint. It would further be advantageous if such a HVAC system and method were still capable of providing optimal control of air quality under normal conditions, that is, under conditions where the HVAC system could provide sufficient heat to keep the discharge air temperature above the DAT setpoint. It would additionally be advantageous if, in order to implement such a system and method, major modifications to existing HVAC systems were not required.
The present inventors have discovered that a cascaded PI control loop control system and a CO2 alarm can be provided within an HVAC system which prioritizes the action of the HVAC system in situations where desired control of both CO2 levels and temperature levels within the room is limited as discussed above. Upon the occurrence of a situation in which the HVAC system is unable to provide any additional heating capacity, the HVAC system switches from a normal heating state to a low limit state.
In the low limit state, a low limit proportional integral (PI) control element provides a control signal to a fresh air damper to control the amount of fresh air being added to form the discharge air. The low limit PI control element bases its output upon the difference between the actual discharge air temperature and the discharge air temperature setpoint produced by a room PI control element, which in turn bases its output upon the difference between the actual room temperature and a room temperature setpoint. Also in the low limit state, the HVAC system continues to provide the maximum amount of heating possible. The HVAC system leaves the low limit state and returns to the normal state once the room CO2 level falls below the CO2 setpoint, such that a CO2 alarm shuts off.
The present invention relates to a method of controlling air quality within a room. The method includes determining a first discharge air temperature setpoint based upon a room temperature setpoint and a first value indicative of an air temperature within the room, and determining a first air flow control signal based upon the first discharge air temperature setpoint and a second value indicative of an air temperature of discharge air being provided into the room. The method further includes controlling an air flow device based upon the first air flow control signal, and maintaining a heating device employed to influence the air temperature of the discharge air at a maximum heating level. The method additionally includes monitoring a level of carbon dioxide within the room to determine whether the level is below a predetermined threshold.
The present invention further relates to a system for controlling air quality within a room. The system includes a damper for controlling an amount of a first type of air to be combined with a second type of air to form an air mixture, and an air pathway to which the damper is coupled, and within which the first and second types of air are combined to form the air mixture. The system additionally includes a coil assembly including a coil that is positioned within the air pathway and a valve, the coil affecting a temperature of the air mixture before the air mixture is output to the room as discharge air. The system further includes a controller coupled to the damper and to the valve, the controller providing first and second control signals respectively thereto. The controller operates in at least a first state and a second state, wherein in the first state the first and second control signals are varied to allow for the control of both an air temperature and a carbon dioxide level within the room, and in the second state the second control signal is maintained at a fixed level.
The present invention additionally relates to a system for controlling the air quality within a room. The system includes a first means for regulating an amount of air being added to the room, a second means for influencing the temperature of the air being added to the room, and a third means for controlling the operation of, and coupled to, the first and second means. The third means operates in at least a first control state and a second control state. In the first control state, the third means can control the first and second means so that both a temperature of the air within the room and a carbon dioxide level within the room are within desired ranges. In the second control state, the third means can control the first means so that at least the temperature of the air within the room is within a desired range.